Based upon the sensor design techniques recently used in batch micro-calorimeters, we have devised an all tantalum stopped flow calorimeter with significantly greater sensitivity and faster response times. The tantalum flow assembly consists of two inlet tubes and one outlet tube which are electron-beam welded to a tantalum mixer. An array of small, fast response thermopiles are located on opposite sides of the outlet tube. The outlet tube contains a volume of 350 micro-liters. A 0.1 mm stainless wire is also attached to the outlet tube for use in calibration. Two such assemblies are used in this differential instrument. The sensing portion of the tubing is mounted in a large heat sink and kept at the core or operating temperature of the calorimeter. As the tubes enter and leave the heat sink, they are routed together so that thermal artifacts propagating down the tubes will appear as common-mode and not differential signals. The resulting instrument has a sensitivity of 1 joule/volt*second and a 10 to 90% step response of 48 seconds.